Method and apparatus for maintaining contact noise in an electrical switching device at a minimum level



May 15, 1962 E. w. GORAL 3,035,141

METHOD AND APPARATUS FOR MAINTAINING CONTACT NOISE IN AN ELECTRICAL SWITCHING DEVICE AT A MINIMUM LEVEL Filed Feb. 6, 1961 CONDUCTIVE MATERIAL PLASTIC r COMPRISING COPOLYMER VINYL 3 CHLORIDE AND VINYLIDENE CHLORIDE INVEN TOR. ERNEST W. GORAL WJ'W ATTORNEY United States Filed Feb. 6, 1961, Ser. No. 87,350 6 Claims. (Cl. 200-166) The present invention relates to methods and means for maintaining contact noise in an electrical switching device at a minimum level.

In step by step telephone switching exchanges, contact noise is sometimes produced due to the presence of metallic oxides on the surface of the wires in the wire banks. It has been observed that the presence of a high degree of moisture in the ambient atmosphere within these exchanges causes a considerable reduction of this contact noise. It is thought that this moisture increases the electrical conductivity of the oxide film. One method of maintaining a high degree of humidity in the ambient atmosphere would utilize air conditioning apparatus which, of course, is relatively expensive. The present invention involves an extremely inexpensive method and means for converting the aforementioned metallic oxides situated on the wire banks into hygroscopic metallic chlorides which absorb moisture from the ambient atmosphere. This conversion of the metallic oxides into hygroscopic metallic chlorides greatly reduces contact noise.

Accordingly, it is the object of the present invention to provide an inexpensive and extremely efiective method and apparatus for reducing contact noise in an electrical switching device.

Other objects, features, and the attending advantages of the present invention will become apparent with reference to the following specification and drawings in which:

FIGS. 1 and 2 disclose one method of carrying out the present invention.

Referring now to FIG. 1 of the drawing, a brush 1 is disclosed adjacent contact members 2, 3 and 4 which may be wires in a telephone switching exchange. Portion 6 of brush 1 is made of an electrically conductive material such as copper. A second portion 7 is disclosed which is composed of a plastic containing copolymer vinyl chloride and vinylidene chloride. Apparatus (not shown) is provided for moving brush 1 over contact members 2, 3 and 4.

The metallic oxides which form on the surfaces of contact members 2, 3 and 4 and brush 1 and which are responsible for contact noise, are eliminated in the wear track of the switching device in the following manner: Just after nose portion 8 of brush 1 rides over contact 3, plastic portion 7 of brush 1 rides over contact member 3, as shown in FIG. 2. This action causes minute particles of the plastic to be rubbed oif brush 1 and to be deposited upon contact member 3. It has been found that when the plastic portion 7 is molded during manufacturing, hydrochloric acid is released so that this acid is distributed throughout plastic portion 7. As the minute plastic particles are deposited on contact member 3, the acid attacks the metallic oxide layer thereby to ate form metallic chloride salts and water. Unlike the metallic oxides, the salts are hygroscopic and absorb moisture from the surrounding air. Also, metallic chlorides, unlike the metallic oxides, are water soluble and dissolve in the water produced by the aforementioned chemical action and in the water absorbed from the air. This results in the formation of a relatively non-irregular layer which has a considerably higher electrical conductivity characteristic than the metallic oxide layer and, accordingly, contact resistance and noise is markedly reduced.

Of course, the present invention is in no way restricted to step by step telephone switches but may be utilized in any electrical switch or distributor where contact noise is to be maintained at a minimum level. The copolymer vinyl chloride and vinylidene chloride is readily moldable, is inexpensive and is commercially available. This material is marketed under the trademark Saran by Dow Chemical Company. It should be noted that the plastic material wears at a slow rate so that brush 1 may pass over the contact members hundreds of thousands of times without completely wearing away plastic portion 7. Minute particles of hydrochloric acid which are distributed within plastic portion 7 are continuously deposited upon the contact members so that the metallic oxides which continuously form upon the contact members are continuously converted into hygroscopic metallic salts to reduce contact noise over the entire lifetime of the switching device.

While there has been disclosed what is at present considered to be the preferred embodiment of the invention, other modifications will readily occur to those skilled in the art. It is not therefore desired that the invention be limited to the specific arrangement shown and described and it is intended in the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a switching device, a first contact member including a first portion of electrically conductive material and a second portion including polyvinyl chloride, one or more second contact members, and means for causing both said first and said second portions of said first contact member to contact said second contact members.

2. In a switching device, a first contact member including a first portion of electrically conductive material and a second portion including copolymer vinyl chloride and vinylidene chloride, one or more second contact members, and means for causing both said first and said second portions of said first contact member to contact said second contact members.

3. In a switching device, a first contact member including a first portion of electrically conductive material and a second portion including polyvinyl chloride, one or more second contact members, and means for sequentially causing both said first and said second portions of said first contact member to contact said second contact members.

4. In a switching device, a first contact member including a first portion of electrically conductive material and a second portion including copolymer vinyl chloride and vinylidene chloride, one or more second contact members, and means for sequentially causing both said first and said second portions of said first contact member to contact said second contact members.

5. A switching device comprising, a first electrical contact member, one or more second electrical contact members, means for causing said first electrical contact member to contact said second electrical contact members, and means for periodically depositing small quantities of copolymer "inyl chloride and vinylidene chloride on said second contact members.

6. A switching device comprising, a first electrical contact member, one or more second electrical contact members, means for causing said first electrical contact mem- (5, her to contact said second electrical contact members, and means for periodically depositing small quantities of polyvinyl chloride on said second contact members.

References Cited in the file of this patent UNITED STATES PATENTS 

